Light scanning devices of the above-mentioned type are used e.g. for a spatially resolved fluorescence examination of a sample. For this purpose, the scanning light is produced in the form of a single beam by means of the light-emitting device, which is a laser in most cases, said scanning light being then directed onto the sample. By means of the scanning unit, e.g. in the form of tilting mirrors with two orthogonal tilting axes or axes of rotation in the optical path of the light beam, the beam can be rastered over the sample. The scanning light excites on the surface of the sample or in the sample the generation of secondary light, e.g. in the form of fluorescent light. This secondary light is collected via an imaging optics and detected on a detection unit. Since the scanning unit irradiates, in a precisely definable manner, a respective specific spot on the sample in dependence upon the position of the tilting mirrors relative to one another and relative to the sample, a locally dependent statement with regard to the respective property of the sample can be made by means of the detection unit detecting the intensity of the secondary light.
Since the spatial resolution is already obtained by the scanning unit, the detector device according to the prior art is a simple spot detector without spatial resolution which only detects the presence or the absence of secondary light emission independently of the point of its generation on the sample. However, after the irradiation of a specific scanning point on the sample, the irradiation of the next scanning point on the sample must be delayed until the electric signal produced by the secondary light in the photodetector has been recognized and read out and until the photodetector has been re-initialized for the next measurement. Even if a fast read-out electronics is used, this waiting time represents an undesirable delay in a fluorescence examination of a comparatively large sample to be scanned.
The scanning time for measuring the whole sample depends on various additional parameters, such as the size of the angular field on the sample, the scanning increment, the spot size of the scanning beam on the sample, the integration time of the detection unit, the scanning or mirror velocity of the scanning unit as well as the desired signal-to-noise ratio. When samples with dimensions in the centimeter range are scanned with a high spatial resolution by a scanning beam focussed to a few micrometers, the scanning times of conventional scanning devices are in the range of minutes to hours. Such long scanning times are, however, a great problem for the operation of scanning devices of this kind.